1. Field of the Invention
The present invention relates to the structure of a light emitting diode (LED), and more particularly to the structure of a surface emitting LED with a window layer and a conductive transparent oxide layer for obtaining high brightness.
2. Description of the Prior Art
AlGaInP alloy technology has been used for making light emitting diodes (LEDs) of wavelength ranging from about 550 to 680 nanometers by adjusting the aluminum to gallium ratio in the active region of the LEDs. Further, metalorganic vapor phase epitaxy (MOVPE) is used to grow efficient AlGaInP heterostructure devices. A conventional LED contains a double heterostructure of AlGaInP, which includes an n-type AlGaInP cladding layer formed on an n-type substrate of GaAs, an active layer of AlGaInP formed on the n-type cladding layer, and a p-type AlGaInP cladding layer formed on the active layer.
For efficient operation of the LED, injected current should be spread evenly in the lateral direction, so that the current will cross the p-n junction of the double heterostructure of AlGaInP uniformly to generate light evenly. The p-type AlGaInP cladding layer, which is grown by MOVPE process, is very difficult to dope with acceptors of a concentration higher than 1E18 cm.sup.-3. Further, hole mobility (about 10 to 20 cm.sup.2 *v/sec) is low in p-type AlGaInP semiconductor. Due to these factors, the electrical resistivity of the p-type AlGaInP layer is comparatively high (about 0.3-0.6 .OMEGA.-cm normally), so that current spreading is severely restricted. Consequently, current tends to concentrate, and is often referred to as a current crowding problem.
One technique to solve the current crowding problem is disclosed by Fletcher et. al. in U.S. Pat. No. 5,008,718. The structure of the proposed LED is shown in FIG. 1, and is fabricated with a back electrical contact 10, a substrate of n-type GaAs 12, a double heterostructure of AlGalnP 14, a window layer of p-type GaP 16, and a front electrical contact 18. The double heterostructure of AlGaInP 14 mentioned above includes a bottom cladding layer of n-type AlGaInP 140, an active layer of AlGaInP 142, and a top cladding layer of p-type AlGaInP 144. The window layer 16 should be selected from materials that have a low electrical resistivity so that current can spread out quickly, and have a band gap higher than that of the AlGaInP layers so that the window layer 16 is transparent to light emitted from the active layer of AlGaInP 142.
In an LED for generating light in the spectrum from red to orange, AlGaAs material is selected to form the window layer 16. The AlGaAs material has the advantage of having a lattice constant compatible with that of the underlying GaAs substrate 12. In an LED for generating light in the spectrum from yellow to green, GaAsP or GaP material is used to form the window layer 16. It is a drawback of using the GaAsP or the GaP material that their lattice constants are not compatible with those of the AlGaInP layers 14 and the GaAs substrate 12. This lattice mismatch causes a high dislocation density that produces less than satisfactory optical performance. In Applied Physics Letter, vol 61 (1992), p. 1045, K. H. Huang et. al. discloses a similar structure having a thick layer 16 of about 50 .mu.m (or 500000 angstroms) in thickness. This structure provides a three-times illuminance efficiency than an LED without a window layer, and two-times illuminance efficiency than an LED with a window layer of about 10 .mu.m in thickness. The fabrication of this structure unfavorably requires two different processes of metalorganic vapor phase epitaxy (MOVPE) for growing the double heterostructure of AlGaInP, and vapor phase epitaxy (VPE) for forming the thick window layer of GaP 16, thereby increasing manufacturing time and complexity.
FIG. 2 shows another prior art LED, which is disclosed in U.S. Pat. No. 5,048,035. In this figure, the layers that are not changed in appearance from the structure of FIG. 1 are labeled with the same reference numerals. The LED of FIG. 2, in addition to the structure of FIG. 1, is fabricated with a current-blocking layer of AlGaInP 20 on a portion of the double heterostructure 14, and a contact layer of GaAs 22 between the window layer 16 and the electrode 18. The current-blocking layer 20 is arranged at a position where it is in alignment with the front electrode 18 and thus current is spread out laterally by the current-blocking layer 20. Two MOVPE processes are disadvantageously required in fabricating this structure, i.e., forming the heterostructure 14 and the current-blocking layer 20 by a first MOVPE, followed by a photolithography technique to define the area of the current-blocking layer 20, and forming the window layer 16 by a second MOVPE.
FIG. 3 shows a third prior art LED disclosed in U.S. Pat. No. 5,359,209. In this figure, the layers that are not changed in appearance from the structure of FIG. 1 are labeled with the same reference numerals. The LED of FIG. 3, in addition to the structure of FIG. 1, is fabricated with an additional p-type window layer of GaAs 30 between the heterostructure 14 and the p-type window layer of GaP 16. Although the window layer of GaAs 30 has good conductivity with carrier concentration of 10.sup.19 cm.sup.-3 or more, the structure induces a light absorption phenomenon due to the fact that the energy gap of GaAs is substantially less than that of AlGaInP in the active layer 142.
FIG. 4 further shows a prior art LED disclosed in U.S. Pat. No. 5,481,122. In this figure, the layers that are not changed in appearance from the structure of FIG. 1 are labeled with the same reference numerals. The window layer of GaP 16 in FIG. 1 is now replaced by a p-type contact layer 40 and a conductive transparent oxide layer 42 in FIG. 4. Indium tin oxide (ITO) is preferably used for forming the conductive transparent oxide layer 42, which has high transparency rate at about 90% in the range of visible light. Further, its electrical resistivity (about 3.times.10.sup.-4 .OMEGA.-cm) is about 1000 times smaller than that of p-type AlGaInP, and about 100 times smaller than that of p-type GaP. However, the optimal thickness of about 1000.about.50000 angstroms does not provide a good condition for effectively emitting light, thereby confining the illuminance efficiency of the LED.